The harvesting of process tomatoes is done almost exclusively with mechanical harvesting machines in the state of California where the vast majority of U.S. process tomatoes are grown. The mechanical tomato harvesting process involves mechanically digging up the tomato plants, transferring them to a shaker and mechanically shaking the tomatoes from the vines. Consequently, a large number of green tomatoes, dirt clods and rocks are collected along with acceptable tomatoes.
Tomato processing plants that receive the harvested tomatoes from the fields and the California Department of Agriculture have established inspection standards that process tomatoes must meet. To determine if tomatoes delivered to a processing plant meet the established standards, random samples are taken from each load of tomatoes delivered. The samples are inspected to be sure that the load does not contain excessive numbers of green tomatoes, dirt clods, rocks, defects and other extraneous material. It is therefore necessary to sort the rejects from the good tomatoes during harvesting in the field in order to guarantee that each load of tomatoes delivered to a processing plant meets or exceeds inspection standards. This makes it necessary to do a high volume sorting operation while harvesting since harvesters operate at an average rate of 25 tons of tomatoes per hour. The sorting of process tomatoes in the last few years has been done more and more by the use of high volume electronic sorting apparatus mounted directly on the harvester.
The basic principle of operation for the electronic sorting machines is to drop the tomatoes to be inspected off the end of a feed conveyor that is on the harvester. Just after the objects leave the feed conveyor, they are illuminated and inspected in flight by an electro-optical device which looks at certain spectral wavelengths of reflected light and rapidly makes a decision to either keep or reject the inspected tomatoes and other objects. The flow of inspected tomatoes and other objects off the conveyor passes in front of a reject mechanism which can be extended so as to divert the trajectory of unacceptable objects over a dividing baffle and through a chute to the ground. Acceptable tomatoes go to a further conveyor for loading onto a truck.
One commonly used method for sorting tomatoes is to measure the red and green reflectance of the tomato and to compare one color signal with the other. When the green signal exceeds the red signal, the color is classified as green. When the red signal exceeds the green signal, the color is classified as red. This test gives a very reliable red/green sort most of the time. However, in the northern parts of California where the majority of process tomatoes are grown, there is a significant percentage of dark green tomatoes which have very low red and green spectral reflectances in the range of 10%. These tomatoes give relatively low sorting signals as well as very small voltage differences between red and green color signals. This leads to uncertainty and frequent misgrading of green tomatoes.
An improved electronic sorting method is disclosed in U.S. patent application Ser. No. 765,716, now U.S. Pat. No. 4,095,696, by J. R. Sherwood. This method involves measuring the red reflectance of a tomato and comparing the red color signal with a reference signal IR.sub.1 in the near infra red range at 800 nanometers (nm) to get a relative measurement of red content of each tomato. That is, if the red signal exceeds the IR.sub.1 signal (800 nm) the color will be classified as red, and if the IR.sub.1 signal exceeds the red signal, the color is classified as green. The sorting system of the above-mentioned Sherwood application also includes means for distinguishing between vegetable matter and nonvegetable matter. This test allows the system to detect dirt clods and rocks. The system detects nonvegetable matter by comparing the reference IR.sub.1 signal at 800 nm with a second infra red signal IR.sub.2 in the near infra red region at 990 nm. Tomatoes cause a dip in light reflectance around 990 nm while dirt clods and rocks do not. By comparing the IR.sub.1 and IR.sub.2 signals, the presence of rocks and dirt clods may be detected. That system also compared one of the infra red signals against a bias signal to detect the presence of an object at the inspection position.
The above-described red/800 nm color test is extremely effective for sorting out dark green tomatoes because of their characteristic of having relatively low green spectral reflectance compared to their 800 nm IR.sub.1 reflectance. However, this method has the shortcoming that whitish type green tomatoes have very high spectral reflectance of around 80% in the visible spectrum, especially in the red and green spectral regions. Unfortunately, reflectance of the whitish-green tomatoes in the 800 nm band does not increase proportionately. This means that whitish-green tomatoes give red/800 nm reflectance ratios that approach those of acceptable tomatoes. This results in occasional misgrading of whitish-green tomatoes.
A whitish-green tomato is one that has at least a spot of whitish coloring on its skin and which usually is too immature to be acceptable.